Written by CandC
As I have found out, much to my surprise, there are many different measures of the same thing in use in the Nautical world. These measure can be confusing, but also a great way to make a geocache more interesting. I have found two sources of information that are particularly helpful, though some times contradictory: The Admiralty Manual of Navigation, by the U.K.’s Royal Navy and The American Practical Navigator, by the U.S. National Imagery and Mapping Agency and known sometimes as “Bowditch”.
Here’s how the units breakdown:
| Unit | Admiralty (U.K.) | Bowditch (U.S) | Note: |
| International nautical mile (nm) | 1852 m | 1852 m | US Dept of defence uses 6076.11549 ft with a yard to metre conversion of 1x=0.9144m exactly. (From Bowditch) |
| Geographical Mile | 1855.4 m | 6087.08 ft (1855.52 m) | The official definition of this unit is according to both sources “1 minute of arc along the equator.” The admiralty figure given here uses the 1924 spheroid. |
| US Survey Mile | N/A | 5280 ft(1609.5m) | Used on rivers and inland waterways. |
| Sea Mile | 1842.9 m at 0d 1852.3 m at 45d | Same as nautical mile | Defined by the admiralty as the length of 1 minute of arc of latitude. Figures are given for the 1924 spheroid. |
| Statute Mile | 1760 x, 5280 ft | N/A | Note this is the same as a US Survey mile. |
| Cable | Depends on Latitude (aprox. 200x) | 185.2 m | Defined by both the Admiralty and Bowditch as one tenth of a sea mile. Note that sea miles differ according to the source. The Admiralty points out that a cable is most often approximated as 200 yards. [Apparently, a cable once considered to be 120 fathoms] |
| Fathom | Not given | 6 ft (1.82898m) | |
| Shackle | [12.5 fathoms (22.862m)] | [15 fathoms 27.435m] | Neither of these measures is given by either source, however these measures are still used by mariners, to the best of my admittedly limited knowledge. |
Note:
- Figures in (round brackets) are my conversion and have been converted using 1ft=0.30483m.
- Text in square brackets is from The Sailor’s Illustrated Dictionary by Thompson Lenfestay, and is attributed to a particular country.
